A swimming sea slug

This is the spectacular Sagaminopteron ornatum, sometimes known as the "Batwing Slug" because of its greatly enlarged wing-like parapodia. It was first found in Sagami Bay, Japan which gives rise to its name Sagaminopteron. The Gastropteridae are specialised Bubble Shells related to Philine and Chelidonura. Unfortunately we know little about their biology. To my knowledge no one has identified the food of any species. Some tropical species are usually found on particular sponges but the radular morphology suggests they are carnivores eating small active invertebrates rather than rasping sponge feeders. I have found nothing recognisable in stomach contents. I suspect that they feed on small flatworms which leave no recognisable stomach contents, but that is only a guess. I would be glad to hear from anyone who has any feeding observations on any gastropterid.

One of the most fascinating features of the gastropterids is their swimming behaviour. The genus name Gastropteron can be loosely translated as "winged stomach", which is quite apt. Like most opisthobranchs, they are normally benthic crawlers, but when disturbed they can vigorously flap their greatly enlarged parapodia and remain suspended in the water column for some minutes. In the composite picture alongside, the various stages in the swimming "stroke" are illustrated. .... Bill Rudman.